The conventional switching mode power supply circuit is, as illustrated in FIG. 1, composed in such way that the output terminal of the rectifier 1 to which the alternating current is supplied, is connected to the collector of the switching transistor TR1 through the primary winding T11 of the transformer T1, to the diode D1 and the base of the said switching transistor TR1 through the resistor R1. The secondary winding T12 of the transformer T1 is connected to the base of the switching transistor TR1 through the resistor R2 and the condenser C1 and the secondary winding T15 is jointly connected through the rectifier 2 and the control part 5. The secondary winding T13 of the transformer T1 is connected to the remote control part 6 through the rectifier 3 and the relay RL1 is connected to the control terminal ClS of the remote control part 6. The secondary winding T14 of the transformer T1 is connected to the load 7 through the switch RL11 of the relay RL1 and the rectifier 4.
According to the composed conventional switching mode power supply circuit, when the alternating current AC is applied, the alternating current is, after rectified in the recitifer 1, applied to the primary winding T11 of the transformer T1 and at the same time to the base of the transistor TR1 through the resistor R1. Therefore, the blocking oscillation circuit comprising transistor TR1, the primary and secondary windings T11, T12 of the transformer T1, resistors R1, R2, condenser C1 and diode D1 begin to oscillate as the switching transistor TR1 turns on and off. Accordingly, the voltage produced in the primary winding T11 of the transformer T1 is induced and output to the secondary windings T12-T15.
At this moment, the electric current flows to the base of the switching transistor TR1 via the resistor R2 and the condenser C1 by the voltage induced to the secondary winding of the transformer T1. In a case where the switching transistor TR1 turns on, the electric current flowing to the primary winding T11 of the transformer T1 is gradually increased. When the current reaches hfe times the base electric current of the switching transistor TR1 (wherein hfe is the current amplification factor), the switching transistor TR1 turns off.
The voltage induced to the secondary winding T13 of the transformer T1 is rectified in the rectifier 3 and applied to the remote control part 6 as an operating power so as to operate the remote control part 6 by the remote control signal. Thus, when the remote control part 6 is driven, a control signal is output to the control terminal CS to which operates the relay RL1 and the switch RL11 so that relay RL1 is short-circuited. Consequently, the voltage induced to the secondary winding T14 of the transformer T1 is rectified in the rectifier 4 through the switch RL11 of relay RL1 and the voltage is applied to the load 7 as an operating voltage which is expressed in the following formula: ##EQU1## Wherein Vo is the operating voltage applied to the load 7,
Ro is the impedance of the load 7, PA1 L is the impedance of the primary winding T11 of transformer T1, PA1 T is the ON/OFF cycle of the switching transistor TR1, PA1 Ton is the time for which the transistor TR1 stays in an ON state, and PA1 Vi is the output voltage of the rectifier 1.
In the above-described formula, the operating voltage Vo applied to the load 7 is varied depending on the impedance Ro of the load 7 and the output voltage Vi of rectifier 1. The secondary winding T15 is connected to the secondary winding T14 so that the voltage induced to the secondary winding T15 is varied in accordance with any changes of the voltage induced to the secondary winding T14. The voltage induced to the secondary winding T15 is rectified in the rectifier 2 and then applied to the control part 5. At this time, the control part 5 controls the base current of the transistor TR1 so that both the output voltage of the rectifier 2 and the predetermined reference voltage become equal in their magnitudes.
Accordingly, in the event that less base current from the transistor TR1 flows due to such control of the control part 5, the decrease in base current results in a shortening in the time for which the electric current flowing to the primary winding T11 of the transformer T1 reachs hfe times the base current of the transistor TR1 when the transistor TR1 turns on, and the time for which the transistor TR1 stays in ON-state is shortened. While in the event that more base current flows, the time in which the base current reaches hfe times, is lengthened, and the time in which the transistor TR1 stays in an ON state is also lengthened. Because an ON/OFF cycle of the transistor TR1 is controlled according to its base current, the magnitude of the voltage induced from the primary winding T11 to the secondary winding T14 of the transformer T1 is constant regardless of the impedance Ro of the load 7 and the output voltage Vi of rectifier 1, that is, the magnitude of the input alternating current, and a constant operating voltage is applied to the load 7.
The maximum capacity of electric power supplied to the power supply circuit is about several hundred watts, the voltage range of the alternating current is 90-270 V, and the frequency used exceeds to 20 KHz.
However, the above-mentioned conventional switching mode power supply circuit has the following problems depending on the standby state for applying the operating power only to the remote control part 6 and the driving state for applying the operating power to both the remote control part 6 and the load 7.
Supposing that the use of electric power for the remote control part 6 is 2W and the electric power use of the load 7 is 100 W, a large difference of about 30:1 in the ratio between Ton/.sqroot.T takes place in which an alternating current of 90 V input and an operating power of 102 W is supplied to both the remote control part 6 and the load 7, and Ton/.sqroot.T in which the alternating current of 270 V is input and 2 W is supplied only to the remote control 6.
Consequently, in a case where the operating power is applied only to the remote control part 6, the ON-time of the switching transistor TR1 becomes very short, and the frequency is also increased.
The voltage produced in the primary winding T11 is proportion to the winding ratio of the primary and secondary windings T11. T12 of the transformer T1 is induced to the secondary winding T12, and the magnitude of the voltage induced to the secondary winding T12 when the switching transistor TR1 turns on, and the values of the resistor R2 and the condenser C1 are set to output 100 W, the electric power use of the load 7, at the time of a 90 V alternating current input. Therefore, the voltage induced to the secondary winding T12 when an alternating current AC of 270 W is input, becomes three times greater than to a case where the alternating current of 90 V is input, and the base current of the transistor TR1 is also increased by three times.
Accordingly, in the event that the operating power is applied only to the remote control part 6 and not to the load 7 when the alternating current of a high voltage is input. There are disadvantages in that because it is very difficult to control that the control part 5 controls the base current of switching transistor TR1, the output voltage is not maintained constantly. As the frequency is greatly increased, it is required to use a switching transistor TR1 having an excellent switching characteristic, and in a case where the switching characteristic of the switching transistor TR1 is not excellent, a parasittic oscillation, etc. takes place and thereby the operation of circuit becomes very unstable, and the switching transistor TR1 is destroyed.